Because of process variations in the fabrication of Gm-C or MOS-C circuits, it is difficult to match filter circuits to the accuracy required in many applications. One solution to this problem is to provide on-chip tuning of the Gm-C and MOS-C circuits by comparing the resistors and/or capacitors of the latter-mentioned circuits with an accurate reference resistor and/or capacitor. In the event of a mismatch a comparison circuit generates a control voltage which changes the Gm-C or MOS-C resistance and/or capacitance until a match is obtained. In MOS-C circuits the width and length of the MOS transistors of the filters can be matched to those of a tuning circuit so that the various factors such as fabrication tolerances, operating temperature variations, and aging will affect both MOS transistors the same so that these effects tend to cancel.
It is possible to automatically tune RC products rather than just resistance values by utilizing either an accurate external clock or an accurate on-chip clock. A reference circuit is formed in which the resistors and capacitors of the filter are ratio matched to those of the reference circuit so that the RC products within the reference circuit are also stabilized. The reference circuit is either a duplicate of the filter or of one basic cell of the filter. A phase comparator compares the phase of the reference "filter" output with that of the clock. The comparator outputs a control voltage which adjusts the reference "filter" so that the phase difference is at a predetermined value. This causes the RC products within the reference filter to attain fixed, predetermined values.
Alternately, a voltage controlled oscillator can be used as the reference circuit. A voltage controlled oscillator consists of a phase detector, a loop filter and a voltage controlled oscillator to tune the filter. A phase comparator compares the output of the oscillator with the clock and produces a control voltage which causes the oscillator output to track the clock. The RC products of the oscillator and filter are stabilized by the control voltage as is the frequency response of the filter. Typically phase lock loops require a significant amount of circuitry and hence chip area. In addition they introduce noise at the reference frequency provided for the phase lock loop.
Tuning circuits which feature external reference resistors require initial adjustments and do not compensate well enough for both resistance and capacitance. Often such circuits have offsets which cause frequency tuning errors. Phase locked loop circuits occupy a relatively large area. In addition, it is difficult to build high frequency voltage controlled oscillators that are well-matched to an associated filter
U.S. Pat. No. 5,245,646 issued to Jackson et al. discloses a tuning circuit having an integrator with an RC time constant which is proportional to the RC time constant of a filter. The filter uses discrete capacitors and is not continuous. Moreover, series resistors in the tuning circuit and filter limit the frequency to below about 10 MHz.
U.S. Pat. No. 5,124,593 issued to Michel discloses a continuous time filter tuning circuit which applies only to MOSFET-C or MOS-C filters. If the capacitor is not large enough it will create wandering of the MOSFET-C device.
Accordingly, it is an object of the invention to provide an improved training circuit for use in tuning a filter circuit time constant to be a scaled multiple of the RC time constant of a reference filter circuit.